1. Field of the Invention
The invention relates to a novel paint resin based on a modified high-pressure polyethylene (LDPE) with special adhesion characteristics and the use thereof for powder coating.
2. Description of the Related Art
There has been no shortage of attempts to protect structures and components, particularly metal substances, from the effect of corrosive enviroments by coatings of reasonably priced and chemically resistant polymers, such as polyethylene or polyvinyl chloride. However, the adhesion characteristics of these polymers are inadequate with the result that corrosion creep sets in on the metal surfaces under the coatings and longterm protection is impaired.
Copolymers from ethylene and unsaturated carboxylic acids are already known (U.S. Pat. No. 3,520,861). The acid content is described therein as 0.1-20% by weight; yet only with an acrylic acid content of not less than about 4% by weight are adhesion characteristics achieved which are adequate for using such copolymers for coating and laminates.
For economic reasons, the acid copolymer is often employed as primer only for the more reasonably priced polyolefin. For example, GB 2,097,809 describes a steel pipe coating produced with a 0.25 mm film thickness of a primer consisting of a propylene-acrylic acid copolymer, and 2.5 mm film thickness of a topcoat consisting of a mixture of polypropylene/polybut-1-ene.
An operation using greater amounts of unsaturated carboxylic acids makes high demands in respect of corrosion resistance on the polymerization equipment. For this reason an earlier study was carried out (U.S. Pat. No. 3,132,120) using the tert.-butyl esters of unsaturated carboxylic acids present as copolymerized units instead of the unsaturated carboxylic acids. After the polymerization the carboxyl group must then be freed by pyrolysis with elimination of isobutene. However, here, too, higher acid contents, for example 4.2% by weight of methacrylic acid, are necessary to achieve adequate adhesion characteristics.